This invention relates to weldments of austenitic stainless steel components and method and, more particularly, to weldments and method for assembling stainless steel grids to ducts for fuel assemblies for liquid-metal-cooled nuclear reactors.
A fuel assembly for a liquid-metal-cooled nuclear reactor includes a plurality of cylindrical fuel rods supported within a duct by spacer grids. The ducts and grids have typically been fabricated of Type 316 stainless steel. The grids must be accurately located and permanently attached to the duct. In past practices, the grids have been attached to the ducts using the gas tungsten-arc welding (GTAW) process wherein the two pieces to be joined are clamped by a suitable fixture and the weld made by experimentally optimized choice of welding parameters. Such welds have been successful using 18Cr-8Ni stainless steel.
The aforementioned copending application Ser. No. 245,487 addressed the problem of producing a sound weld joint in an advanced austenitic stainless steel alloy containing more nickel and less chromium than previously employed. The welds were successfully accomplished in the manner as described by utilization of the delta-ferrite-forming insert.
It is desirable in some cases to use grid straps having a thickness such as 0.02 inch (0.5 mm) and a relatively heavy duct wall having a thickness such as 0.12 inch (3 mm). With this thickness of duct wall, heat transfer through the relatively thick member to the much thinner grid strap presents a major problem. With advanced high-nickel, stainless steel alloys such as alloy D9, weld results became erratic.
In the process described in aforementioned copending application Ser. No. 245,487, welding was accomplished from the outside surface of the relatively heavy-walled duct. In order to improve heat transfer to the thin grid member, it was attempted to move the welding torch to the inside surface of the duct so that heat transfer initially occurred through the thin grid member. While satisfactory welds could generally be made utilizing Type 316 stainless steel, with advanced alloys such as alloy D9, severe cracking occured at the interface between the joint members and into the spot weld or weld nugget. The nature of the defects were classified in metallurgical terms as "solidification cracking".